Strike-slip Duplex Research Articles

Depositional and geotectonic history of the Gala area, eastern Southern Uplands, Scotland

AbstractThe Ordovician and Silurian successions between Falahill and Galashiels encompass six flysch-dominated formations: the Upper Ordovician Portpatrick and Shinnel Formations representing the Leadhills Group, the Llandovery Mindork, Garheugh, and Buckholm Formations together comprising the Gala Group, and a formation indeterminate of age within the Hawick Group. Southward ensialic andesitic volcanic arc and northward low- to medium-grade sialic sources contributed sediment, whilst ophiolitic and subduction-related sources made minor contributions. Deposition took place firstly, in a SE-migrating back-arc basin bordering the northerly source, the Laurentian continent. Subsequent NW-directed underthrusting led to formation out of the back-arc basin of an imbricate thrust stack which migrated southeastwards. Ultimately a foreland successor basin formed ahead of the rising thrust stack.Flysch units are typically associated with linear outcrops of Moffat Shales which are the loci of major steep SE-translating reverse faults, two of which participate in a late-stage sinistral strike–slip duplex with large-scale imbrication. The faults divide the succession into a sequence of tectonostratigraphic blocks, successively younger to the SE. At least six of the ten blocks customarily recognised in the Southern Uplands, Blocks 3–8, are represented, some of which coincide with single or complete formations.

Formation of gold veins and breccias during dextral strike-slip faulting in the Mesquite mining district, southeastern California

The Mesquite mining district in southeastern California is presently the second largest gold producer in California. The district comprises two subparallel orebodies of Oligocene age that before mining were unconformably buried by a thin veneer of late Tertiary sedimentary rocks and minor basalt and Quaternary alluvium on the south piedmont slope of the Chocolate Mountains. This range forms part of the eastern margin of the Salton trough and is within an area cut by numerous, but now-inactive, strands of the Neogene San Andreas fault system. Extensive postmineral faulting, inferred to be related to the evolution of the San Andreas fault system since the middle Miocene, has disrupted, offset, and rotated parts of the orebodies. This dismemberment, occurring primarily along northeasterly striking, left-lateral, oblique-slip faults, has resulted in a stepped pattern of economic mineralization in map view on the piedmont. Mineralized faults were reactivated during this event; the reactivated faults have normal, thrust, and strike-slip displacements that accommodated relative rotations of blocks between the northeast-striking oblique-slip faults. The postmineral faulting converted gold-bearing quartz and carbonate veins to breccia or to clayey gouge without any accompanying hydrothermal mineralization. A palinspastic reconstruction of the orebodies sharpens their original geometric form.The two gold orebodies, thus reconstructed, consist of the large, Big Chief and Vista open pits and the small, lower economic-grade Cherokee and Rainbow open pits. Each orebody consists of upward-diverging, moderately to steeply dipping veins, breccias, and mineralized faults that cut amphibolite facies metamorphic rocks and granitoids of Jurassic and Cretaceous age. Published geochronologic data indicate the orebodies formed in the Oligocene. Northwesterly striking dextral strike-slip faults and subparallel veins and northerly striking extensional faults and veins are the dominant mineralized structures in the restored orebodies. These two ore-related fault sets mutually crosscut each other and they formed contemporaneously. Fault movement, vein formation, and mineralization were episodic and record progressive heterogeneous simple shear strains. Throughout the orebodies, the geometry of mineralized structures in their present map and cross-sectional views requires that they formed synchronously with episodic dextral strike-slip faulting under conditions of approximately north-south shortening and east-west extensional strains. Structural criteria include mineralized faults with subhorizontal to gently plunging slickensides; identifiable Riedel, conjugate Riedel, and P shears that define a dextral strike-slip fault system of mutually cross-cutting and braided faults; the character and relative orientation of extension veins with respect to the strike-slip faults; composite dilational jogs defined by the distribution of economic grades; flower structures; and strike-slip duplexes. The structural evidence is independent of scale and is present at the scale of (1) the bench wall, (2) the individual open pits, and (3) the entire mining district. The two orebodies are interpreted to have formed in areas of relative extension between subparallel dextral strike-slip faults in a combination of fault jogs and strike-slip duplexes. This structural setting for the Mesquite mining district cannot be reconciled with models that purport to link the formation of the orebodies to extensional deformation and detachment faulting during the Oligocene.

Structural controls in the distribution of gold at How Mine, Bulawayo, Zimbabwe

The How mine is situated in the southeastern quadrant of the Bulawayo greenstone belt within sediments and volcaniclastics of the late Archean Dacitic Greenstone Formation of the Upper Bulawayan Group. Sited on the northern limb of a northeast-verging syncline, the mine comprises several ore zones which occur as an en echelon array of parallel, steeply plunging linear shoots. These shoots are confined within an extensional right-stepping, northerly trending strike-slip duplex. Faulting has followed preferred lithologic contacts.Mineralization is dominated by a pyrite-gold association and occurs as disseminations within altered mylonites (felsites) and tuffs, as fabric replacement within tuffs, and associated with veins in siltstones and iron-formations. Accessory magnetite in tuffs and magnetite layers in iron-formation are replaced by pyrite, and ore textures suggest an epigenetic origin to the mineralization. Alteration is widespread and dominated by carbonation, sulfidization, and propylitic alteration.Permeability within the duplex was generated by slip and dilation of the synclinal axialplanar cleavage, which lies at a high angle to the strike of the duplex. Early ductile deformation is overprinted by later brittle faulting which cuts early mineralization. The linear ore zones parallel the intersection lineation between the preexisting cleavage and the duplex fault-shear zones. In the north area of the mine, ore zones are developed within the fault-bounded tuff unit (e.g., the north 180 ore zone), but in the south the ore zones parallel the Hanging-wall fault-shear zone and transgress major lithologic contacts.

Resurgent strike-slip duplex development along the Hitra-Snåsa and Verran Faults, Møre-trøndelag fault zone, Central Norway

Lineament analysis of the ENE-WSW Verran and Hitra-Snåsa Faults of the long-lived Møre-Trøndelag Fault Zone (MTFZ), has revealed the presence of a complex array of anastomosing faults and fractures. This has the geometric configuration of a major, dextral, strike-slip fault zone and compares well with characteristic patterns of braided wrench-faulting, e.g. the San Andreas Fault System of California. The fault array recognized in conjunction with the Verran Fault is clearly post-Caledonian and is considered to define a dextral, strike-slip duplex system. Associated with the parallel Hitra-Snåsa Fault are Riedel-like structures which tend to point to an earlier component of sinistral movement. Rock products present along the Hitra-Snåsa Fault and its secondary faults comprise mylonites, hydrothermally altered rocks and small-scale recrystallized breccias. Along the main Verran Fault, evidence of late polyphasal deformation is seen in several discrete episodes of brecciation, hydrothermal alteration, and locally pervasive prehnite and stilbite veining. The fault structures occurring along the Hitra-Snåsa and Verran faults are thought to have originated as a sinistral fault system of late Devonian age, especially for the Hitra-Snåsa lineament. Subsequently, strike-slip movement reversed in sense and shifted locus towards the Verran Fault system during the late Jurassic or early Cretaceous. During this strike-slip reversal, some earlier fractures related to the sinistral system were rejuvenated within the stress field of the evolving dextral duplex system. Strike-slip displacements of a similar age are known from fault complexes on the Norwegian continental shelf and in the northern North Sea. The regional picture indicates that the MTFZ was almost certainly linked to the fault systems of northern Scotland, prior to the opening of the Viking Graben.

Strike-slip faulting along the Church Stretton Lineament, Old Radnor Inlier, Wales

The Old Radnor Inlier comprises Precambrian sediments unconformably overlain by Wenlock age limestones and shales. Its structure is dominated by steep strike-slip faults. Major NE-SW faults probably bound the inlier and are linked by more northerly faults to form a strike-slip duplex. Strike-slip faults within the inlier match a Riedel shear system, with observed dextral offsets on a NW striking set and implied sinistral offsets on the major inlier-parallel faults. Subordinate dip-slip faults are anomalously oriented, but may be Riedel shears reactivated during progressive sinistral transtension across the inlier. Pre-Wenlock deformation is recorded by the tilting of the Precambrian sediments and by some faults that do not cut the unconformity. On regional grounds, this deformation might be Ashgill (late Ordovician) or late Precambrian-early Cambrian in age. The strike-slip component now predominating was imposed in post-Wenlock time, probably during the regional sinistral transpression of the Acadian (Late Caledonian) event in Early Devonian time. Later Variscan and Mesozoic components cannot be ruled out.

Tectonic controls of Ordovician arc and marginal basin volcanism in Wales

Ensialic destructive plate margin volcanism persisted in Wales from late Tremadoc to Caradoc times. Late Tremadoc uplift and erosion in NW Wales is attributable to onset of subduction and development of a frontal ridge system (fore-arc). South-east of this, in the vicinity of the Harlech Dome and in SW Wales, more localized uplift was associated with the development of arc volcanoes at a volcanic front. In North Wales the arc volcanism occurred in a regime of E-W crustal extension and was centred on a deep crustal discontinuity. During Arenig times the arc terrane was transgressed. Subsequently, marine conditions, with local and temporary emergence, persisted through Arenig to Caradoc episodes of marginal basin volcanism. While there was widespread and persistent subsidence in Wales during development of the marginal basin, the volcanism occurred mainly along complex and relatively narrow grabens which were sites of repeated channelling of magmas and pronounced subsidence. The grabens mark the sites of deep and steep crustal discontinuities (‘fractures’), one of which controlled the earlier arc volcanism. Changing sites of volcanism in Wales can now be seen as due to transfer of active extension from one fracture to another. There is evidence that the fractures were present by Tremadoc times, and they almost certainly formed before development of the Cambrian marine rift-basin. They are most likely to have been strike-slip faults or shear zones, and their pattern in Wales indicates that they may represent part of a complicated deep-crustal strike-slip duplex system. The volcanically active grabens of the marginal basin are considered to reflect upward propagation of splay faults (flower structure) which channelled magmas through the cover during extension across the fractures. The fractures and associated grabens strongly influenced the location and orientation of zones of relatively high strain that developed during Late Caledonian compression and transpression. In North Wales the volcanically active grabens trended broadly N-S, oblique to the exposed NE-SW basement tectonic grain, and dominantly E-W extension seems to have persisted throughout the Ordovician volcanism. While there is considerable uncertainty concerning Ordovìcian plate tectonic configurations and displacements, the extension is best explained by invoking a N-S convergence vector between the oceanic and over-riding microcontinental plates, with resultant sinistral shear stress in the latter and sinistral transtensional displacements across the arc and marginal basin. The model highlights the suspect nature of the relationship between the Welsh Ordovician terrane and others to the north.

The Dent Fault System, northern England—reinterpreted as a major oblique-slip fault zone

The Dent Fault System forms part of the Pennine Boundary Fault System in northern England and consists of a series of vertical to steeply west-dipping, anastomosing fault strands generally striking 010°–020°. Maximum throw on the faults occurs in the south, where an easterly downthrow juxtaposes Carboniferous and Lower Palaeozoic rocks. The amount of easterly downthrow decreases northward, and westerly downthrow occurs along its northernmost extension. An east-facing and slightly overturned monocline is dissected by the fault strands along the length of the fault system, irrespective of the amount or sense of displacement across the zone. A number of features suggest that the observed structural relationships are not simply the results of E-W directed, post-Carboniferous compression as previously thought. The most significant are: (1) the variable amount and sense of displacement along the length of the sub-vertical fault system, (2) east-directed reverse faults exhibiting oblique- and strike-slip slickenside striations, that locally may form a partial- or half-flower structure, (3) folds developed both sub-parallel and en echelon to the main fault zone, (4) the existence of conjugate sets of shear fractures, (5) the occurrence of structures analogous to strike-slip duplexes, and (6) regional considerations. Instead, it is suggested that the fault system represents an Early Palaeozoic lineament, reactivated during Carboniferous and into early Permian time. It accommodated both the formation and N–S directed shortening (inversion) of early Carboniferous basins and later NE–SW directed extension during the formation of the Vale of Eden half-graben, by oblique-slip movement along its length. Younger movements may also have occurred but are difficult to document and are not thought to be large.

Pseudotachylyte-bearing strike-slip duplex structures in the Fort Foster Brittle Zone, S. Maine

Detailed (1:60 scale) mapping of the Fort Foster Brittle Zone in the mylonitic Rye Formation of southernmost Maine has revealed the intricate internal duplex structure of a system of probable Paleozoic-age dextral strike-slip faults that have produced abundant pseudotachylyte and minor breccia. The internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab-duplex structures. Individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer-parallel slip surfaces along which frictional melts were produced. These slab-duplex structures are interpreted as zones of displacement transfer between long, overlapping, layer-parallel en échelon strike-slip fault surfaces. Contractional duplexes develop layer-parallel compressional structures that tend to shorten and thicken the fault-bounded slabs by the formation of lateral ramps and conjugate faults, kinks and asymmetric folds. Extensional duplexes develop layer-parallel stretching and thinning by the formation of oblique dextral shears, high-angle conjugate pairs and localized fault breccias. The production of pseudotachylyte by friction melting along layer-parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events. The rupture structures developed during these events may be characteristic of fault structure and mechanics at near-focal depths in a strike-slip seismogenic zone.

Strike-slip duplexes

Strike-slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip-slip, thrust and normal, fault systems. They are evident in map view rather than in vertical sections. Examples of duplexes are cited from both active and ancient systems and from theoretical and physical models. Duplexes may form at bends on strike-slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip-slip faults. However some may form, and many may initiate, as non-sequential ‘Riedel’ fractures at fault offsets or on straight fault segments. This process is more marked than in dip-slip systems where primary anisotropy such as bedding exerts more control on fault geometry. Strike-slip duplexes may be shunted along the fault system parallel to the regional slip vector. However, duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system. These factors mean that no section through a strike-slip system should be expected to area balance. The faults of strike-slip duplexes and imbricate fans may root in kinematically necessary low-dip faults or may converge downwards and appear in vertical sections as flower structures.